Just wanted to report that things are now working fine. I rewired everything, adding another neutral and another earth connector block linked to the existing ones, so that no wires are doubled up on any earth or neutral connector. All wire ends are now terminated in isolated ferrules. I tightened all connections carefully. I checked for the correct signal on the switched live with a multimeter, before connecting switched live to the boiler.
The problem on the last step must have had to do with a poor connection. I read up a lot on possible issues, but I simply cannot explain my scenario: It all worked fine, I then did nothing else than adding a new boiler cable with switched live included, it started off fine until suddenly all components stopped. There was no issue with the power supply from the mains. Same scenario again twice after - starting up fine then switching off all components. Eventually, even removing the boiler live did not fix it. I ended up removing the boiler cables altogether, and then connected them again without the switched live. I do not think however that the addition of the switched live was in any way relevant, as it works fine now in exactly the same wiring constellation.
If anybody has any further input on what kind of poor connection and scenario could have caused this failure, it would certainly make me feel much better, and might help others who may read this thread in the future!
Thanks again for all the advice. Fingers crossed, but it looks as if I now have a safer set-up, with no more external live feeds that are not isolated by the boiler switch coming into the wiring centre from programmer and UFH controller, plus the addition of wireless switching of all components!